Can a debt collector access my credit card limits and are they obligated to supply me with an original bill?

In March a derogatory mark was posted to my credit report for a debt I was unaware of--a doctor's bill. To my knowledge I've never received any notice of this debt, and so I contacted the debt collector who attempted to intimidate me by listing off the credit limits on my credit cards and told me I could afford to pay the bill, then hung up on me. I've since then requested a copy of the original bill multiple times--as I've never seen it and was unaware of it. They recently sent me an e-mail with a statement saying I owed $925 but no itemized bill. Every time I request this bill, they tell me they've either sent it to an address I haven't lived at in years or they ignore my request and tell me they can settle the debt for $x.xx. I'm really unsure of what to do--I'm not unwilling to pay this debt if it is legitimate, but I am unwilling to pay without having a copy of the bill to be able to verify that it's legitimate. The debt collector is Capio Partners.